Info

800 1200 1600 2000 2400 ^ 2800 Temperature (° F)-►

Fig. 10.1. Relative Material Temperature Limits

Polymer and Metal Matrix Composites Ceramic Matrix Composites

Strengthening Toughening

Strengthening Toughening

Fig. 10.3. Stress-Strain for Monolithic and Ceramic Matrix Composites

Debonding

Fiber Pull-out
Fig. 10.4. Crack Dissipation Mechanisms

mechanisms of debonding and fiber pull-out are shown in Fig. 10.4. For these mechanisms to be effective, there must be a relatively weak bond at the fiber-to-matrix interface. If there is a strong bond, the crack will propagate straight through the fibers, resulting in little or no energy absorption. Therefore, proper control of the interface is critical. Coatings are often applied to protect the fibers during processing and to provide a weak fiber-to-matrix bond.

Carbon-carbon (C-C) composites1 are the oldest and most mature of the ceramic matrix composites. They were developed in the 1950s by the aerospace industry for use as rocket motor casings, heat shields, leading edges, and thermal protection. It should be noted that C-C composites are often treated as a separate material class from other ceramic matrix composites, but their usage and fabrication procedures are similar and overlap other ceramic matrix composites. A relative comparison of C-C with other ceramic matrix composites is given in Table 10.1. For high temperature applications carbon-carbon composites offer exceptional thermal stability (>4000° F) in non-oxidizing atmospheres, along with low densities (0.054-0.072lb/in.3). Their low thermal expansion and range of thermal conductivities provides high thermal shock resistance. In vacuum and inert gas atmospheres, carbon is an extremely stable material, capable of use to temperatures exceeding 4000° F. However, in oxidizing atmospheres, it starts oxidizing at temperatures as low as 950° F. Therefore, C-C composites for elevated temperature applications must be protected with oxidation resistant coating systems, such as silicon carbide that is over-coated with glasses. The silicon carbide coating provides the basic protection, while the glass over-coat melts and flows into coating cracks at elevated temperature. In addition, oxidation inhibitors, such as boron, are often added to the matrix to provide additional protection.

Ceramic matrix materials include the element carbon, glasses, glass-ceramics, oxides (e.g., alumina - Al2O3) and non-oxides (e.g., silicon carbide - SiC). The majority of ceramic materials are crystalline with predominately ionic bonding,

Table 10.1 Carbon-Carbon and Ceramic Composite Comparison

Carbon-Carbon

Continuous CMCs

Discontinuous CMCs

Exceptional High Temp Mech

Excellent High Temp Mech

Excellent High Temp

Properties

Properties

Mech Properties

High Specific Strength and

High Specific Strength and

Lower Specific Strength

Stiffness

Stiffness

and Stiffness

Low to Moderate Toughness

Low to Moderate Toughness

Was this article helpful?

0 0

Post a comment